Flat panel displays are widely used in a variety of applications, including computer displays. One type of device suited for such applications is the field emission display. Field emission displays typically include a generally planar substrate having an array of projecting emitters. In many cases, the emitters are conical projections integral to the substrate. Typically, the emitters are grouped into emitter sets where the base of the emitters are commonly connected. A conductive extraction grid is positioned above the emitters and driven with a voltage of about 30 V-120 V. The emitter sets are then selectively activated by coupling the bases to ground. Grounding the emitter sets creates electric fields between the extraction grid and the emitters of sufficient intensity to extract electrons from the emitters and also provides a current path between the emitters and ground.
The field emission display also includes a display screen mounted adjacent the substrate. The display screen is formed from a glass plate coated with a transparent conductive material to form an anode biased to about 1-2 kV. A cathodoluminescent layer covers the exposed surface of the anode. The emitted electrons are attracted by the anode and strike the cathodoluminescent layer causing the cathodoluminescent layer to emit light at the impact site. The emitted light then passes through the glass plate and the anode where it is visible to a viewer.
The brightness of the light produced in response to the emitted electrons depends, in part, upon the rate at which electrons strike the cathodoluminescent layer, which in turn depends upon the magnitude of current. The brightness of each area can thus be controlled by controlling the current flow to the respective emitter set. By selectively controlling the current flow to the emitter sets, the light from each area of the display can be controlled and an image can be produced. The light emitted from each of the areas thus becomes all or part of a picture element or "pixel."
Typically, current flow to the emitter sets is controlled by controlling the voltage applied to the bases of the emitter sets to produce a selected voltage differential between the emitters and the extraction grid. The electric field intensity between the emitters and the extraction grid is then the voltage differential divided by the distance between the emitters and the extraction grid. The magnitude of the current to the emitter set corresponds to the intensity of the electric field.
One problem with the above-described approach is that the response of emitter sets to applied grid and emitter voltages may be non-uniform. Typically, this is caused by variations in the separation between the emitters and the extraction grid across the array, which causes differences in the electric field intensity for a given voltage difference. Often these variations result from variations in the diameter of apertures into which the emitters project, which in turn, are caused by processing variations. For a given voltage differential between the emitters and the extraction grid, the brightness of emitted light may vary according to the location of the emitters.